1. Field of the Invention
The present invention relates to a fuel injection system wherein the fuel tank is located below the electronically controlled fuel injection apparatus.
2. Description of the Related Art
Conventionally, an electronically controlled fuel injection apparatus is known to supply fuel to an engine. In general, an electronically controlled fuel injection apparatus is located below a fuel tank, and there is a case that the fuel tank and the electronically controlled fuel injection apparatus are connected by a fuel supply passage, and fuel is supplied to the electronically controlled fuel injection apparatus from the fuel tank by gravity-drop utilizing head difference.
When ambient temperature of an engine rises, vapor is generated in the fuel in the fuel tank or the fuel supply passage. Then, fuel which contains vapor is supplied to the electronically controlled fuel injection apparatus. When vapor is contained in the fuel which is injected from the electronically controlled fuel injection apparatus, an appropriate amount of fuel cannot be injected into an intake passage. Then, problems occur, such that specific air fuel ratio cannot be obtained or malfunctions of restarting occur.
It is desirable that vapor is not contained in the fuel which is supplied to the electronically controlled fuel injection apparatus. However, practically, it is impossible to completely prevent vapor from being generated in the fuel which is supplied to an electronically controlled fuel injection apparatus. A conventional configuration to obtain an appropriate air fuel ratio even when fuel which contains vapor is supplied to an electronically controlled fuel injection apparatus is disclosed in Japanese Patent laid-open 2003-42032 (page 3, FIG. 1). In this conventional configuration, a fuel return passage is disposed between an electronically controlled fuel injection apparatus and a fuel tank, and more fuel than the amount to be injected is supplied to the electronically controlled fuel injection apparatus from the fuel tank. Vapor is contained in the supplied fuel at high temperature. Vapor is separated from the fuel inside the electronically controlled fuel injection apparatus, namely, fuel for injection goes downwards and vapor goes upwards. The separated vapor and surplus fuel which was not used by the electronically controlled fuel injection apparatus are returned to the fuel tank through the fuel return passage. In this manner, even when vapor is contained in the fuel which is supplied to an electronically controlled fuel injection apparatus, the fuel to be injected to an engine does not contain vapor and a stable air fuel ratio can be obtained by mixing vapor into the surplus fuel which is returned from the electronically controlled fuel injection apparatus.
Conventionally, with a motorcycle, the fuel tank is disposed between the knees or below the seat, and an electronically controlled fuel injection apparatus is disposed below the fuel tank. Then, fuel is supplied to the electronically controlled fuel injection apparatus from the fuel tank by gravity-drop utilizing head difference. With this fuel supply method utilizing head difference, there is an advantage of cost reduction by not having a fuel supply means to supply fuel to an electronically controlled fuel injection apparatus. Further, there is an advantage that vapor moves upwards inside the fuel supply passage and returns to the fuel tank automatically.
Depending on the purpose or the layout of an engine, there is a case that the fuel tank has to be located below the electronically controlled fuel injection apparatus. In the case that a fuel tank is located below an electronically controlled fuel injection apparatus, fuel has to be transported upwards to the electronically controlled fuel injection apparatus from the fuel tank via a fuel supply passage by utilizing a fuel pump.
In the case that fuel is transported to an electronically controlled fuel injection apparatus from a fuel tank utilizing a fuel pump, fuel transported upwards by the fuel pump returns to the fuel tank via the fuel supply passage when the engine stops, due to lack of stability of a check valve of the fuel pump. Therefore, there arises a problem that fuel supply is delayed when the engine is started or restarted, or that the ease of starting the engine is worsened due to vapor remaining within passages. Further, since the electronically controlled fuel injection apparatus is disposed above the fuel tank, when vapor is contained in the fuel in the fuel supply passage which connects the electronically controlled fuel injection apparatus and the fuel tank, vapor may move upwards through the fuel supply passage, and a large amount of vapor may accumulate in the electronically controlled fuel injection apparatus. Therefore, there arises a problem that fuel containing vapor is injected from the electronically controlled fuel injection apparatus, and an appropriate air fuel ratio cannot be obtained.
The present invention was devised in view of the abovementioned problems. The object is to provide a fuel injection system which can inject an appropriate amount of fuel from an electronically controlled fuel injection apparatus while returning vapor existing somewhere in the fuel passages, even in the condition that the fuel tank is disposed below an electronically controlled fuel injection apparatus.